Project summary: Dysregulated programmed cell death causes major health problems: too little death causes cancer and autoimmunity, and too much death causes bone marrow disorders such as Myelodysplastic Syndromes (MDS) and Alzheimer's disease. The major health problems of MDS arise from the death of critical cells resulting in cardiac stress and infection. In order to maintain hematopoietic function in the face of loss of critical hematopoietic progenitor cells, hematopoietic stem cells must actively cycle, exposing them to mutagenesis and perhaps driving the high rate at which MDS transforms to Acute Myelogenous Leukemia (AML). Understanding mechanisms of cell death regulation will lead to important insights into disease pathogenesis and transformation to malignancy, as well as new targets for therapeutic intervention. There are two major regulatory pathways used to induce cell death: apoptosis and necrosis. While the gene pathways and mechanisms driving apoptotic cell death have been extensively studied, those driving necrosis are less well understood. Moreover, necrosis but not apoptosis triggers an inflammatory response. Necrosis facilitates pathogen clearance during infection, but causes additional tissue damage in acute injuries such as stroke and myocardial infarction. Death receptor activation can induce both apoptotic and necrotic cell death. Exciting recent data indicates that components of the upstream apoptotic signaling pathway, including Caspase 8 and FLIP, inhibit necrosis, suggesting the presence of an early signaling switch to determine cell fate. Multiple lines of evidence placeBid as a component of the upstream death receptor-signaling pathway. We have developed a mouse model in which loss of Bid results in unrestrained bone marrow necrosis and bone marrow failure. Furthermore, we demonstrate that Rip1 kinase is elevated and Bid is decreased in primary MDS samples. Rip1 levels are highest in the RCMD subtype of MDS. Based on our preliminary findings, the central hypothesis of this proposal is that BH3-only Bid plays a critical role in hematopoietic homeostasis, by suppressing necrosis, perhaps by receiving and executing signals downstream of TNF via Caspase 8 (extrinsic apoptosis) and Rip Kinases (necrosis).To test this central hypothesis, we propose to dissect the mechanism by which Bid inhibits necrosis (SA1) by defining Bid's interaction with pro-necrotic signaling complexes. We will further define Bid's impact on cell death signaling through the Rip kinases. To begin to investigate the impact of dysregulated necrotic signaling on bone marrow function (SA2), we will dissect the effect of Bid- mediated apoptosis and necrosis on hematopoietic stem and progenitor cell proliferation, differentiation, and regeneration in our mouse model that reveals unrestrained bone marrow necrosis. (SA3) will investigate necrosis signaling in primary human MDS bone marrow samples to correlate activation of necrosis and outcome in the subtypes of MDS. We will further evaluate the ability of inhibitors of necrosis to prevent bone marrow cell death in MDS samples.